This application claims the benefit of Japanese Application No. 2001-257757 filed Aug. 28, 2001.
The present invention relates to an operation console for producing a tomographic image of a region to be examined based on projection data of the region to be examined, a gantry apparatus for emitting X-rays onto a region to be examined in a subject and generating projection data based on X-rays passing through the region to be examined, an X-ray CT system comprised of a gantry apparatus for emitting X-rays onto a region to be examined in a subject and generating projection data based on X-rays passing through the region to be examined and an operation console for producing a tomographic image of the region to be examined based on the projection data of the region to be examined obtained by the gantry apparatus, methods of controlling the operation console and the gantry apparatus, and a program code and a storage medium.
An X-ray CT system emits X-rays onto a subject (patient), detects the differences of X-ray absorption factors of human tissue in organs, blood, gray matters etc. by a detector, and provides an image (tomographic image) of a cross-sectional plane (a slice plane) of the region to be examined by computer-processing (reconstructing) the differences.
A physician diagnoses the condition etc. of the patient based on the tomographic image of the patient in a predefined region to be examined reconstructed by the X-ray CT system. To this end, the reconstructed tomographic image must have image quality that enables distinction of the differences of X-ray absorption factors of human tissue with good accuracy and that matches the purpose of the examination. To get such image quality, it is necessary to reduce image noise.
The image noise in the X-ray CT system is in general represented by the variance of CT values when a homogeneous material is imaged. (The variance will be referred to as an image SD hereinbelow.) A smaller image SD value corresponds to higher quality of a tomographic image, and a larger image SD value corresponds to lower quality of a tomographic image. To reduce the image noise and obtain a tomographic image with high quality (i.e., to reduce the image SD), the amount of transmitted X-rays that pass through the subject and are detected at the detector must be large; and hence, to obtain a sufficient amount of transmitted X-rays at the detector, the amount of X-rays emitted onto the subject must be large.
Paying attention only to improvement of the image quality and increasing the amount of the X-rays emitted onto the subject, however, undesirably leads to an increase of the exposure dose to the subject. Therefore, in practice, it is necessary to conduct control so that the amount of X-rays emitted from the X-ray tube is the minimum required to secure desired image quality.
The amount of X-rays emitted from the X-ray tube is controlled by the electric current passed to the X-ray tube (which will be referred to as the tube current or mA hereinbelow). Conventional X-ray CT systems are normally provided with functions for controlling the tube current to achieve such minimum X-ray emission, including an auto mA function (automatic tube current controlling function).
The auto mA function in the conventional X-ray CT systems involves: emitting beforehand a predetermined amount of X-rays in a predefined direction onto a predefined region for each subject (such a scan involving emitting beforehand X-rays for each subject to obtain desired data is sometimes referred to as a scout scan); calculating the image SD based on a measurement result with the amount of transmitted X-rays (baseline data) at that time; and establishing an image quality level required for a tomographic image for examination (such a target image quality level is sometimes referred to as a target SD); and calculating the tube current for examination based on the ratio of the image SD and target SD.
The target SD to be established is determined based on the slice thickness and the region to be examined for examination. Specifically, the target SD is determined with reference to a table as shown in FIG. 1. FIG. 1 shows an exemplary table for use in establishing the target SD.
For example, when the region to be examined is the chest and the slice thickness is 10 mm, the target SD value can be set to 5.4 referring to the table in FIG. 1 (in the mode IQ).
By employing the aforementioned tube current calculating method, the tube current matching the region to be examined can be calculated. By such a method, however, the region to be examined is exposed to X-rays corresponding to a constant tube current. Thus, excessive X-rays may be applied to the region to be examined depending upon the thickness thereof in some X-ray emitting directions. Consequently, the patient is exposed to unnecessary radiation.
Therefore, an object of the invention is to reduce the exposure of the subject to unnecessary radiation by controlling the tube current according to the X-ray emitting direction with respect to the region to be examined in the subject.
To attain the object of the present invention, an operation console of the present invention has, for example, a configuration as described below.
Specifically, there is provided an operation console for producing a tomographic image of a region to be examined based on projection data of said region to be examined, comprising: transmitting/receiving device for transmitting to an external apparatus instructions for collecting projection data, and receiving said projection data collected by said apparatus, wherein said external apparatus collects projection data at each scan position in a region to be examined in a subject by emitting X-rays in a predefined direction onto said region to be examined and carrying said subject; first calculating device for calculating a first control value for controlling the amount of X-rays emitted by said apparatus onto each scan position during a scan, based on the projection data at each scan position received by said transmitting/receiving device and the particulars of said instructions; and second calculating device for calculating a second control value for controlling the amount of X-rays according to the emitting angle of said X-rays emitted by said apparatus onto each scan position during a scan; wherein said transmitting/receiving device further transmits to said apparatus instructions for conducting a scan based on said first and second control values.
Said first calculating device calculates said first control value by correcting a control value for controlling the amount of X-rays in collecting projection data based on a target image quality level with respect to a tomographic image based on said projection data at each scan position.
When a tomographic image based on projection data at each scan position is approximated to an ellipse, said first control value controls the amount of X-rays emitted by said apparatus in the major-axis direction of said ellipse.
When a tomographic image based on projection data at each scan position is approximated to an ellipse, said second calculating device calculates said second control value by calculating the ratio of the major and minor axes of said ellipse and using said ratio and said first control value.
When a tomographic image based on projection data at each scan position is approximated to an ellipse, said second control value controls the amount of X-rays emitted in the minor-axis direction of said ellipse.
The particulars of said instructions are the region to be examined, slice thickness, and control value for controlling the amount of X-rays.
Said transmitting/receiving device transmits to said apparatus instructions for conducting a scan that periodically uses said first control value and said second control value.
Said first control value and said second control value represent the tube current passed to a controller for controlling the amount of X-rays when said apparatus emits said X-rays.
To attain the object of the present invention, a gantry apparatus of the present invention has, for example, a configuration as described below.
Specifically, there is provided a gantry apparatus for emitting X-rays onto a region to be examined in a subject, and generating scan data based on X-rays passing through said region to be-examined, comprising: transmitting/receiving device for receiving from an external apparatus instructions for collecting projection data at each scan position in a region to be examined in a subject by emitting X-rays in a predefined direction onto said region to be examined and carrying said subject, transmitting the collected projection data to said apparatus, and receiving from said apparatus scan instructions for controlling the amount of X-rays according to the emitting angle of said X-rays onto each scan position; and control device for conducting control over the amount of X-rays according to the emitting angle of said X-rays emitted onto said region to be examined based on said scan instructions received by said transmitting/receiving device.
Therefore, the present invention can control the tube current according to the X-ray emitting direction with respect to a region to be examined in a subject, and can reduce exposure of the subject to unnecessary radiation.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.